People can guess private keys? Seriously? No one is capable of doing that genius. Many have tried to crack these keys in different ways(Brute force etc), but failed which is why a large portion of BTC is lost basically.
Maybe he was referring to superhumans lol,i don't think normal person can crack that private key combinations
Supercomputers could possibly crack them in the future, but it would still be really difficult.
Well there is no impossible for the technology now thats why we.must have.more security measure on how we.can.prevent this from happening